I think you're going to go through a whole lot of effort for very little gain. Unless you swap out the windows for dual pane versions, insulate the box and tail gate, dual gasket the tailgate to bed seal, you won't really gain much.
I'm not setting out on an Arctic expedition and I know it won't be cozy as home, but little gains can sometimes make HUGE improvements! Houses existed for a long time without double pane windows and dual seal doorways, a lot still don't have them. You are correct about the box though, without some sort of insulation there, insulating the roof does a lot less good!